Dynamics of energy metabolism in the heart will be studied by the new technique of phosphorous nuclear magnetic resonance. The working heart model will be studied as a function of cardiac work and substrate utilization. The regulation of aerobic metabolism by availability of cytoplasmic reducing equivalents and the mediation of metabolic efficiency by creatine phosphate will be evaluated for variations dependent on work load and substrate availability. These questions will be addressed through study of: 1) changes in high-energy phosphate levels across the cardiac cycle, 2) turnover rates of high-energy phosphate with respect to the variables of work and substrate, and 3) turnover rates of high-energy phosphates will be studied with respect to position in the cardiac cycle. The effect of inclusion of various substrates on the regulation of energy metabolism and the role of these substrates in maintaining efficiency in high-energy phosphate utilization and production as cardiac work is increased will be evaluated in detail. In all experiments, mechanical performance will be monitored and correlated with metabolic data.